1. Field of Invention
The present invention relates to radiography technology, in particular to an image reconstruction method for a dual-energy CT image with high-energy X-rays.
2. Description of Prior Art
Computerized Tomography (CT) technique has been widely applied in the fields of medical diagnosis and industrial lossless detection. Demand for the CT technique, including application of a huge number of X-ray imaging-based CT imaging systems (hereafter referred to as XCT system for short), in public security and social safeguard is also growing with the evolution of the society. The XCT systems can be categorized into mono-energy CT and dual-energy CT in terms of the utilized imaging techniques. The mono-energy and dual-energy XCTs, of which the techniques are relatively sophisticated, are mainstream XCTs in reality applications, though multi-energy X-ray imaging technique has made some progress. The mono-energy XCT can obtain information on the internal structure and physical characteristics of an object by reconstructing an attenuation coefficient image of the tomogram of the object, while it cannot distinguish and determine exactly what substance is comprised in the object. On the other hand, the dual-energy XCT can obtain not only information on the attenuation coefficient of the substance within the object but also information on composition of the substance through the reconstruction approach. For example, typical information on composition of the substance is effective atomic number and equivalent characteristic density of a substance. With such information, it is possible to perform substance recognition with a high accuracy and thus to provide an efficient inspection approach in the field of public security.
Dual-energy XCT technology has entered a relatively sophisticated stage and been widely used in the medical imaging and security inspection on small-sized objects. In most of dual-energy XCT systems, X-rays of low-energy, generally below 200 KeV, are commonly used in imaging. The reasons for choosing a low energy spectrum are that: first, it is easier to generate X-rays of low-energy by an X-ray tube, and protection against radiation can be implementation in a simpler way for X-rays of low-energy; second, the attenuation coefficients of different substances vary greatly in this energy spectrum, and thus the image of a substance has a better distinguishability; last, the scanned object is usually of a small size and causes less attenuation to X-rays, which enables the application of dual-energy XCT systems utilizing low-energy X-rays.
In security inspection on bulk cargoes, however, the penetrating ability of X-rays in the lower energy spectrum is far from sufficiency, and thus it is impossible to obtain clear and usable projection data for image reconstruction. In general, X-rays in the energy spectrum of MeV order, ranging from 1 to 10 MeV, are required for X-ray radiography. The conventional low-energy, dual-energy reconstruction methods are not suitable for the MeV-order energy spectrum any more, due to a fundamental fact that, in these method, the interaction between X-rays and a substance will result in only two types of effects, photoelectric effect and Compton scatter, without electron pair effect. On the other hand, in the application of high-energy, dual-energy technology, X-rays generally possess a greater energy than the lowest energy 1.02 MeV produced by the electron pair effect. As a result, the conventional methods based on the above two types of effects cannot be adopted any more, and it is desirable to develop new technology for the high-energy, dual-energy application.